The invention relates to an overexposure indicator for an automatic electronic flash, and more particularly, to an apparatus for indicating the occurrence of an overexposure whenever the light emission from an automatic electronic flash exceeds a proper amount of exposure at which an emission terminate signal is produced.
When taking a picture with an automatic electronic flash, the light emission from an electronic flash is generally controllable over a limited extent, and the emission control of the electronic flash tends toward and overexposure as the distance to an object being photographed reduces or as the diaphragm value increases. By way of example, in an automatic electronic flash of series controlled type, if an emission terminate signal is produced simultaneously with an emission initiate signal, there results a definite or minimum light emission from the electronic flash rather than becoming a zero emission. This is attributable to the nature of a forced commutating circuit including a thyristor of an automatic emission termination circuit associated with the electronic flash. Recently, a TTL (through-the-lens) automatic electronic flash is in practical use which is controlled in accordance with an output from an automatic exposure control circuit of a photographic camera of TTL direct photometry type. This arrangement allows a variety of lightings or automatic exposure control with multiple electronic flashes, by electrically interconnecting the camera and the electronic flash or flashes by means of an electric cord. However, from the standpoint of the minimum light emission, a transmission delay through a signal transmission line between the camera and the emission control circuit of the electronic flash or the use of the multiple electronic flashes causes an increase in the minimum light emission. Specifically, in the multiple flash arrangement, the minimum emission is equal to G.sub.NO L.times..sqroot.N where G.sub.NO L represents the minimum emission of each individual electronic flash and N the number of electronic flashes used. FIG. 1 illustrates a curve L indicating the amount of exposure Ev actually provided by flashlight. As will be seen, due to an increased value of the minimum emission during a photographing operation which utilizes an electronic flash, there results a significant deviation toward an overexposure from a level L.sub.O, representing a proper exposure, as the distance to an object being photographed reduces. It will thus be seen from this Figure that a picture cannot be taken utilizing an electronic flash because of the overexposure for a range of distances P. It will be appreciated that if a facility is provided to allow a photographer to recognize the overexposure, he can then immediately correct the diaphragm or light condition or the distance to an object being photographed to repeat another photographing operation under the flashlight illumination from the electronic flash, thus effectively preventing a failure which might result from the overexposure.
An arrangement is proposed already (U.S. Pat. No. 4,122,465) in which an indication is given that the light emission from an automatic electronic flash represents an overexposure. This arrangement gives an indication of a proper or improper exposure depending on whether an output from a photometric circuit reaches a level where an emission terminate signal is produced, within a very brief period from the initiation of light emission from the electronic flash. However, the length of the "very brief period" from the initiation of emission depends on the emission interval of the electronic flash. By way of example, an electronic flash having a reduced value of guide number has an emission interval of about 2 ms while an electronic flash of an annular bulb form which is used for close-ups has an emission interval which is as long as two or three times the above mentioned value. Thus, the emission interval cannot be uniquely fixed. An overexposure is the result of a complex combination of various factors such as a response lag of an emission termination circuit, a time interval required for the electronic flash to terminate its emission, a residual emission after the termination of the emission which is inherent to an electronic flash of series control type and other factors. Hence, it is undesirable to determine if the exposure is proper or improper solely dependent on the timing of the emission terminate signal from the initiation of the light emission. Such determination would degrade the accuracy. Since the decision of the overexposure is not made on the basis of actual exposure of the image plane, a mere warning or indication of overexposure is insufficient to determine if the exposure is tolerable or intolerable.